Directly heated dispenser cathode

ABSTRACT

A directly heated dispenser-type cathode for microwave tubes and the like, the cathode being formed by a solid electrically conductive substrate, such as tungsten wire, the surface of which has bonded thereto a porous layer of a refractory metal whose pores are impregnated with an electron-emissive material having a low work function, such as barium oxide.

United States Patent [72] Inventors David L. 11111;

Lien S. Wu, Lexington, Ky. [21] Appl No. 619,690 [22] Filed Mar. 1, 1967[45] Patented 1:11.26, 1971 [73] Assignee Semicon Associates, Inc.

Lexington, Ky. a corporation 01' Kentucky [54] DIRECTLY HEATED DISPENSERCATHODE 5 Claims, 2 Drawing Figs.

[52] US. Cl. 313/346, 313/311, 313/341, 313/345 [51] Int. Cl 1101i 1/14,H0111 1/04, H0lj19/06 [50] FieldofSearch 313/311, 346, 337, 341

[56] References Cited UNITED STATES PATENTS 3,160,780 12/1964 Coppola313/346 3,076,916 2/1963 Koppius 313/346 Primary Examiner-John W.Huckert Assistant Examiner -B. Estrin Attorney-Michael Ebert ABSTRACT: Adirectly heated dispenser-type cathode for microwave tubes and the like,the cathode being formed by a solid electrically conductive substrate,such as tungsten wire, the surface of which has bonded thereto a porouslayer of a refractory metal whose pores are impregnated with anelectron-emissive material having a low work function, such as bariumoxide.

DIRECTLY HEATED DISPENSER CATHODE This invention relates generally toelectron-emitting sources, and more particularly to a directly-heateddispenser cathode adapted for use in microwave tubes, such as magnetronsand the like, as well as for other electronic instruments employingelectron-emitting sources.

Most microwave tubes are designed to deliver moderate to high power inthe megawatt range. The electron beams for such tubes are characterizedby high-voltage, high current operation. Among the problems encounteredin microwave tubes'are the limitations imposed by the electron-emittingsource, which may be an indirectly or a directly-heated cathode. Theselimitations are the maximum allowable current density and the effect onthe cathode structure of ion bombardment. In some instances, cathodeswhich require high operating temperatures are not desirable, for thesetemperatures make necessary auxiliary cooling equipment and otherspecial accessories which may be objectionable or costly.

One well-known type of electron-emitting source commonly used inmicrowave tubes is the thoria-type, directly heated cathode. Thiscathode is formed by sintering a pressed mixture of powdered thoriumoxide and powdered tungsten or molybdenum, to form a blank which is thenextruded or otherwise worked. Heating of the cathode is accomplished bypassing a current therethrough which raises the surface temperature toan emissive level. Because of the high work function of the cathodicmaterial, in'operation the surface thereof must be at a temperature ofabout 1600 C. Such directly heated thoria-type cathodes are capable ofproducing high current densities and have advantages over conventionaloxide-coated cathodes wherein a base metal is coated with oxides ofbarium and strontium, for they are able to withstand high-voltage ionbombardment Another well-known type of cathode structure is theindirectly-heated dispenser type, such as are disclosed in prior U.S.Pat. Nos. 2,700,000, 2,813,807, and more recently in U.S. Pat. No.3,118,080, issued on Jan. 4, 1964 to O. G. Koppius. Dispenser cathodesof the indirectly-heated type contain a large amount of lowwork-function semiconductor material impregnated in a porous body, whichis usually of sintered tungsten. During operation, a large amount ofactive metal, such as barium, is produced, which diffuses to theemitting surface and continuously replenishes the active metal which hasbeen evaporated or sputtered. As compared to the standard oxide-coatedcathode, the dispenser cathode is much less sensitive to ionbombardment, and higher current densities are feasible.

Both the thoria-type of directly-heated cathode and the dispenser typeof indirectly-heated cathode are superior in most respects to thestandard oxide-coated cathode for microwave tube applications. Thethoria-type has the advantage over the dispenser type in being easierand cheaper to fabricate. On the other hand, the thoria-type becomesextremely brittle after processing, it contaminates easily at lowtemperature, and does not function well below 1,400 C., whereas thedispenser type is mechanically very rugged, it resists contamination,and can operate successfully at a temperature level as low as 900 C.Among the advantages oflow temperature operation are reduced input powerrequirements, a simplified supporting structure, as well as the use ofless exotic metals in the region surrounding the cathodes.

Accordingly, it is the main object of the present invention to provide adispenser cathode which is directly heated, and which possesses theadvantages both of the directly heated thoria-type cathode and oftheindirectly heated dispenser type, without the drawbacks incident tosuch cathodes.

More specifically, it is an object of this invention to provide adirectly heated dispenser cathode constituted by a solid filamentarywire coated with a porous refractory metal impregnated with an emissivematerial having a low work function.

Among the significant advantages of a directly heated dispenser cathodein accordance with the invention are fast heatup time, fast activation,and low gas evolution, as well as long life. Inasmuch as the directlyheated dispenser cathode may be in filamentary form and makes use of aductile substrate metal, it may be coiled or otherwise handled in themanner of a directly heated filament for use in magnetrons, klystrons,traveling wave and backward wave tubes and in other microwave tubes, aswell as in masers, lasers and in mass spectrometers and inotherelectronic instruments. A further advantage of the invention isthat the dispenser cathode may be fabricated at relatively low costcompared to conventional dispenser cathodes.

Briefly stated, these objects are attained by a fabricating techniquewherein a substrate of a solid, electrically conductive metal has formedthereon a porous layer of refractory metal, which layer is impregnatedwith emissive material. Preferably, the porous layer is formed bycataphoretic coating of the substrate by a reducible oxide mixture,which is thereafter sintered in a reducing atmosphere.

Alternatively, the porous layer may be formed by coating the substratewith a mixture of the refractory metal in powder form and an organicbinder, this being accomplished by spraying, dipping or painting, andthen sintering this coating in vacuum or in a reducing atmosphere.Another method for forming the porous layer is by high-temperaturespraying of the metallic or reducible oxide powder onto the substrateunder a neutral or reducing atmosphere, whichever is appropriate. In allinstances, a porous layer of refractory metal is formed on the substrateand bonded thereto, which layer is then impregnated with the emissivematerial having a low work function.

For a better understanding of the invention, as well as other objectsand further features thereof, reference is made to the followingdetailed description to be read in conjunction with the accompanyingdrawing, wherein:

FIG. 1 is a sectional view of a directly heated dispenser-type cathodein accordance with the invention; and

FIG. 2 is a flow chart of a preferred technique for making this cathode.

Referring now to FIG. 1, a directly heated dispenser cathode inaccordance with the invention is constituted by an electricallyconductive substrate 10 of solid, nonporous metal, having a porous layer11 of a refractory metal, which is sintered and bonded to the substrate,the pores of the layer being impregnated with a low-work functionmaterial.

In practice, the substrate 10 may be a solid wire or a body of anyformed or machined configuration suitable for cathode structures, thewire or body being of tungsten, molybdenum, rhenium, or any suitablecombination of refractory metals. When wire is used, it may have adiameter of 0.001 inch or greater.

Preferably, the substrate is constituted by a tungsten alloy or otherrefractory metal characterized by high strength and good ductility, andhence one which is easy to bend or otherwise handle. Theoretically, itwould be possible to make a directly heated dispenser cathode byimpregnating a porous substrate of electrically conductive material witha work function lowering ingredient, but the resultant cathode would beof low strength and hence difficult to handle. Moreover, the resistivityof the cathode would be unstable because of hightemperatureelectrolysis.

The porous layer on the substrate is preferably formed of refractorymetal oxides, such as W0 M00 or any combination of the reducible oxides,which when sintered in a reducing atmosphere, provides the desiredlayer. The emissive ingredients, which are used to impregnate the porouscoating, may be any of those disclosed in the above-cited patents, suchas alkaline oxides, carbonates or their derivatives, which willdecompose to an oxide. Among the usable impregnates are BaO, CaO, A1 03, SrO, MgO, and/or rare earth oxides, either their carbonates or theirderivatives, in any appropriate emissive combination. The inventionencompasses any known form of emissive material of the type usable in aporous matrix to provide a dispenser-cathode action.

METHOD '1 A tungsten-rhenium wire (3 percent Re by weight) of 0.001 inchdiameter, is first coated with W using an electrophoresis technique. Inthis technique (step I) the coating is applied to the wire by passing itthrough or' dipping it in a suspension of the coating material in asuitable container, an electric field being established in thesuspension by a direct voltage connected between the wire and anelectrode immersed in the suspension or the container itself, if it isconductive. ln plating, the wire acts as an anode and the electrode orthe container as a cathode. The following suspension is used:

W03 50 g. NH4C1 l g. H20 100 cc.

The NH4C1 acts as an ion carrier. Alternatively, MgCl or any other ioncarrier may be used for this purpose. To obtain a coating thickness of.010 inch or more, the voltages and other conditions are:

Voltage 75 v. Current 0.3 amp. Time 5 seconds After the coating isformed, the coated wire is sintered (step 2) in a reducing atmosphere(hydrogen) under temperature conditions not going above 2,300 C. forminutes, or at any time-temperature schedule to reach the desireddensity and to effect bonding of the coating to the substrate. Onesuitable schedule is as follows:

Temperature Time 850 C. 1 minute 1,500 C. 3 minutes 2,l00 C. 3 minutesIf thicker layers are desired, the coated and sintered wire may be againcoated and resintered until the desired thickness is attained.

The impregnant (step 3) is then applied to the sintered porous layer,using electrophoresis, spraying or dipping. After this application, theentire body is then heated in a suitable furnace to a temperature whichis about 100 C. above the melting point of the impregnant but not inexcess of 2,300 C. to cause the impregnant to fill the pores. Onepreferred form of impregnant is BaozCaozAl O having a 4:1:1 mole ratio.The thickness of the layer and the amount of impregnant used depends onthe intended life of the cathode.

Excess surface impregnant is then removed by ultrasonic cleaningtechniques (step 4) which act to dislodge any surface material outsideof the pores in the layer. Abrasive or chemical techniques may also beused for this cleaning step.

A directly heated dispenser cathode made in accordance with the abovesteps, was found to have the following useful properties:

A. Very low gas evolvement (2 X 10-6 mm. hg. 3 minutes at B. Fast heatuptime l,050 C.less than four seconds) C. Fast activation minutes at l,l00C., regardless of total length of filament cathode) D. High currentdensity (average current of two amperes per cm. at 900 C. cathodetemperature, with 50 v. across anode having 0.030 inch spacing.)

E. Long life (minimum of 1,000 hours at 900 C. in l X 10-7 mm. hg.

METHOD 1! This method essentially differs from method I in the manner inwhich the porous layer of refractory metal is formed on the substrate, Aslurry formed of the selected refractory metal,

such as tungsten or molybdenum powder, mixed with an organic binder inan amount to obtain a desired degree of fluidity, is applied to thesubstrate by dipping, painting or spraying. A suitable slurry forthis'purpose is composed of gms. of tungsten, 35 gms. ofacetone/amyl-acetate, and 5 gms. of

The coating thus formed is then sintered m a neutral or METHOD IllHigh-temperature spraying may be effected by single-pass mixing ofemissive material with the refractory metal powders to obtain thedesired composition, and using a flame temperature range of l,200 C. tol,800 C. onto the substrate. Alternatively, multiple-pass,high-temperature spraying may be used wherein the metal powders oroxides go through areducing carrier gas to form a porous layer at thetemperature range of 1,600 C. to 2,300 C., the emissive material thenbeing sprayed at 100 C. above its melting point, and below 2,300

Thus in the dispenser-cathode structure in accordance with theinvention, the substrate acts not only mechanically to support thedispenser components, but also as an electrical heater therefor. 1n wireform, this directly heated dispenser cathode may be handled as athoriated filament without however en countering the drawbacks ofsuch'filaments.

While there have been shown and described preferred embodiments ofdirectly heated dispenser cathodes in accordance with the invention, andvarious techniques for fabricating such cathodes, it will be appreciatedthat many changes and modifications may be made therein without,however, departing from the essential spirit of the invention as definedin the annexed claims.

We claim:

1. A directly heated dispenser cathode structure comprismg:

a. a solid substrate of electrically conductive material, said substratebeing formed of tungsten wire capable of being coiled;

b. a layer of porous material bonded to said substrate and formed ofrefractory metal, said layer material being selected from a classconsisting oftungsten, molybdenum, tungsten-rhenium andmolybdenum-rhenium;

c. an emissive material constituted by a semiconductive material havinga low work function impregnating the pores of said layer; and

(1. means to pass electrical current through said substrate to heat thecathode structure to its operating temperature.

2. A cathode as set forth in claim 1, wherein said substrate is atungsten-rhenium alloy, the percentage of rhenium being about 3 percentby weight.

3. A cathode as set forth in claim 1, wherein said substrate is formedmainly of molybdenum.

4. A cathode as set forth in claim 1 wherein said emissive material isBaO:CaO:Al O having a 4: l :1 mole ratio.

5. A cathode as set forth in claim 1, wherein said porous layer has athickness of at least .001 inch.

Patent No. 3, 558,966 Dated January 26, 1971 Inventor(s) David L- andLien S. WU,

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Claim 1 should read as follows:

1. A directly heated dispenser cathode structure com; ing: a. a solidsubstrate of electrically conductive material, s substrate being formedof tungsten wire capable of being coiled; b. a layer of porous materialbonded to said substrate and formed of refractory .metal said layermaterial being selected from a class consisting of tungsten, molybdenum,tungsten-rhenium and molybdenum-rhenium; c. an emissive materialconstituted by a semiconductive material having a low work functionimpregnating the pores of said layer; and d. means to pass electricalcurrent through said substrate heat the cathode structure to atemperature at which an act metal is produced which passes through thepores of said la to the emitting surface thereof to continuouslyreplenish the active metal which has been evaporated or sputtered frcthe surface.

Signed and sealed this 25th day of May 1 971 (SEAL) Attest:

EDWARD M.FLETCEER,JR. WILLIAM E. SGHTUYLER, JR. Attesting OfficerConmissioner of Patents FORM PO-iOSO (10-69) USCOMM DC 603.)

1. A directly heated dispenser cathode structure comprising: a. a solidsubstrate of electrically conductive material, said substrate beingformed of tungsten wire capable of being coiled; b. a layer of porousmaterial bonded to said substrate and formed of refractory metal, saidlayer material being selected from a class consisting of tungsten,molybdenum, tungstenrhenium and molybdenum-rhenium; c. an emissivematerial constituted by a semiconductive material having a low workfunction impregnating the pores of said layer; and d. means to passElectrical current through said substrate to heat the cathode structureto its operating temperature.
 2. A cathode as set forth in claim 1,wherein said substrate is a tungsten-rhenium alloy, the percentage ofrhenium being about 3 percent by weight.
 3. A cathode as set forth inclaim 1, wherein said substrate is formed mainly of molybdenum.
 4. Acathode as set forth in claim 1 wherein said emissive material isBaO:CaO:Al2O3, having a 4:1:1 mole ratio.
 5. A cathode as set forth inclaim 1, wherein said porous layer has a thickness of at least .001inch.